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Illustration, not a medical image · Fluid flow simplified · Evidence notes for this view

Structures

06 · selected

Cerebellar tonsil

The two lowest parts of the cerebellum. In Chiari, they extend below the opening at the base of the skull and are often pointed (peg-shaped) rather than rounded. They really sit just to either side of the midline and often descend unevenly; the model shows them at their lowest point, where MRI measures them.

The tonsils extend below the opening, at least about 5 mm by the usual definition. In one study of 160 people with Chiari I, the average was about 12 mm, but it ranged from 5 to 25 mm, and the number alone doesn't decide who needs surgery. They are often pointed rather than rounded. The fluid space behind them is often crowded, and in many people fluid flow slows or is blocked there.

TypicalChiari IWith syrinxAfter surgery

Chiari I

Tonsils extend below the opening and often crowd the fluid space; in many people flow slows there.

Illustration, not a medical image · Fluid flow simplified · Evidence notes for this view